Numerous methods are conventional by which it is possible to determine the position of the rotor of a synchronous motor in order to permit the electrical commutation of the motor, even without position information from a position-measuring instrument. Many of these methods are based on complicated measurements of the inductances of the primary-circuit coils or the measurement and evaluation of the electromotive counterforce. Such methods make it possible to operate a synchronous motor completely without a position-measuring instrument.
If for the start-up or when putting the synchronous motor into operation, the desire is merely to determine the rotor position, for instance, to determine an offset between the position-measuring instrument and the rotor, or to operate the motor to a reference point, then simpler methods are sufficient for determining the rotor position. Of importance for the operation of a synchronous motor may be actually the position of the vector of the magnetic moment of the rotor, but in simplified terms, the position of the rotor is spoken of in the following.
European Published Patent Application No. 1 085 650 describes a method, according to which initially a first motor phase is energized. Based on the resulting movement direction of the rotor, it is possible to electrically pin down the rotor position to an angular range of 180°. If a second phase which is situated centrally in this range is then energized, the rotor position can be electrically pinned down to an angular range of 90° based on the resulting direction of rotation of the rotor. With each further step, this range is halved, so that the method can be aborted when the desired exactitude has been achieved. A disadvantage in this method, however, is that relatively large movements of the rotor must be brought about before the resulting direction of rotation can be unequivocally detected with the aid of a position-measuring instrument. If these movements are selected to be too small, then grooving forces and machine vibrations can lead to a false result. The rotor position is then no longer correctly detected. However, larger movements are problematic and are not allowed in all cases, for instance, when a synchronous motor is driving a tool that is already engaged with a workpiece. Another disadvantage of this method is that the current necessary for generating a movement of the rotor becomes increasingly greater, the narrower the angular range becomes. Namely, if the magnetic field generated by the current vector is already nearly parallel to the rotor, then, given a constant current, the torque generated becomes increasingly smaller. The accuracy of the method is thus limited by the maximum possible current.